xander, willow, and oddly enough, giles were the easiest ive ever had to write. i dont know why. maybe, for willow and xander at least, because they've been in almost every episode, so im used to hearing the way they talk...but it wasnt as easy for me to write for buffy. i dont know if this came through in anything ive posted on BB, but their voices seem very clear in my head and their dialogue comes semi-naturally to me.
anya, for me, is easy to write, except when she's still a demon. writing for anyanka has posed as a great challenge for me in the past. im too used to her the other way i suppose. also oz. he doesnt say a lot, and what he does say is supposed to be deep and/or important. also, when you write for him, you sometimes have to communicate a lot of feelings into just a few words, which i cant seem to do.
for the writers, i think they probly wouldve gotten the hang of all the characters, making them all pretty easy. they might also have had trouble with oz though.